This invention relates to an electronic flash unit having an exposure calculator which uses the principle of triangulation. More particularly, the invention relates to an exposure calculator having a presettable scale for automatically calculating the appropriate aperture or "F stop" to be preset on the camera for a desired subject.
Range finding devices which use the principle of triangulation for determining the distance between the camera and a subject are well known in the art. Some of such previous devices have used a pendulous indicator which when released will swing due to gravitational forces to cause an indicating needle to align relative to a predetermined foot scale and thereby calculate the camera-to-subject distance directly in feet. Other improvements also have been made which permit the range finding device to accommodate for variations in the type of flashbulb or light used, the type of reflector used, the type of film used and the shutter speed setting of the camera. However, in most instances, the operator has been required to convert the camera-to-subject feet dimension into a correct F stop number corresponding to the particular film used.
According to the present invention, the exposure calculator uses a similar principle of triangulation, but automatically converts the camera-to-subject distance to a particular F stop setting corresponding to the speed of the film being used. The F stop indicating scale is rotatably mounted on the housing of the flash unit to permit a predetermined relative setting of the F stop scale for a particular ASA film speed number. After this setting, the flash unit is aligned through a viewer with the base of the subject to be photographed. An actuating button is depressed to free the pendulous indicator which rotates relative to the F stop scale due to the influences of gravity. The actuating button then is released to lock the indicator into position thereby indicating the correct F stop setting for the camera lens. The flash unit also is provided with an auxiliary light to aid the user in aligning the flash unit with the base of the subject. This light is illuminated only while the actuating button is depressed.
It is an object of this invention to provide a new and improved electronic flash unit having a built in exposure calculator including the foregoing improvements which will be efficient in use, durable, and which can be manufactured for a reasonable cost.
In accordance with the present invention, the flash unit is mounted within a housing with an internal power source. The exposure calculator device also is mounted within the housing, including sighting means on the housing for alignment in the desired angular orientation. The sighting means includes auxiliary light means connected to the power source for aiding in visual alignment. A pendulous indicator is rotatably supported in the housing for movement, due to the influence of gravity, relative an F stop scale for indicating the correct exposure. The F stop scale is rotatably mounted with respect to the housing to permit adjustment of the scale to acommodate varying types of films. An actuating button is connected to the auxiliary light means for turning the light means off and on and for selectively stopping reltive movement of the indicator with respect to the F stop scale. The actuating button is movable between a depressed position, wherein the light is illuminated and the indicator is free to rotate according to the dictates of gravity, and a released position, wherein the light means is not illuminated and the indicator is locked relative to the F stop scale for observance by the user.